A number of methods are known for the preparation of polymer-polyol dispersions and, more specifically, for the preparation of dispersions of polyureas and the like in polyols. The stability of such dispersions is generally reflected in small particle size and low tendency for phase separation. Various steps have been taken over the years to improve the stability of such dispersions.
U.S. Pat. No. 3,325,421 to Muller describes a method for the preparation of a polyurea dispersion in a solvent such as a polyol. This involves the reaction between an organic isocyanate and a primary or secondary diamine, hydrazine or difunctional hydrazide in the polyol as the reaction medium. U.S. Pat. No. 4,093,569 to Reischi et al. describes the preparation of such polyurea dispersions using substantial quantities of water, particularly to provide high solids content dispersions. The quantity of water present is stated to be of decisive importance in determining the particle size, the particle size distribution and final viscosity of the dispersion. U.S. Pat. No. 4,089,835 to Konig et al. describes preparing this type of product using a continuous high-performance flow mixer so as to provide nonsedimenting dispersions of polyureas and pol-yhydrazo-dicarbonamides or corresponding mixed polyaddition products with viscosities of less than 2500 cps. at a resinous solids content of about 10% by weight.
The preparation of this type of product is predicated on the belief that, due to the different reactivity between the hydrogen atoms bonded to the nitrogen atoms and the polyisocyanate and the hydroxyl groups of the solvent and the polyisocyanate, a preferential reaction occurs between the hydrogen atoms bonded to the nitrogen atoms and the isocyanate groups with such a sharp differentiation that a stable dispersion of a polyurea or the like in the polyol solvent can be prepared without substantial reaction with the solvent with suitable stoichiometry.
The apparent need to resort to the use of water and/or particular mixing equipment, as well as probably the emphasis upon relatively high reaction temperatures, to prepare satisfactory products of this type is certainly an indication of the difficulties involved in stabilizing a product of this type. Stabilization is provided by the graft species resulting from the condensation reaction of a polyol molecule with a growing polymer chain. Stabilization of the system in this fashion can be problematic since, as has been previously noted, the reaction rate of the diisocyanate and the amine used will typically be orders of magnitude faster than the rat of reaction between an isocyanato radical and a hydroxyl group. This apparently results in a limitation as to the amount of polymer which may be used, and perhaps other process limitations, if a satisfactorily stable dispersion is to be obtained. Reischi, et al. and Konig, et al. recognize this problem, and accordingly suggest including emulsifying substances in the polyaddition reaction so that the stability of the dispersion will be increased. These emulsifying substances include linear polyethers having an average molecular weight of 300 to 4000 and carrying isocyanate groups or amino or hydrazide groups at both ends of the chain or preferably only at one end.
In the basic process described in these latter two patents, the need to use the processing aids identified seems to be tailored to a rather specific type of polymerization. More particularly, the utilization of relatively large amounts of water added to the polyol and the polyamine used should provide two phases, typically an emulsion of some sort. The use of the high performance flow mixer is apparently designed to provide water phase droplets of relatively small size to thus control the polymer particle size which is formed by polymerization of the polyisocyanate with the polyamine, followed by eventual phase separation to form the dispersion. The use of relatively high reaction temperatures is apparently intended to decrease the difference in reactivity with isocyanate radicals between the amino groups and the hydroxyl groups. Obtaining satisfactory dispersion stability must accordingly be predicated upon satisfactorily optimizing what comes down to an emulsion polymerization type of reaction, perhaps even involving interfacial polymerization.